Another thing that relates to tombstoning is the age, type, and
condition of your paste, as well as the time delay between pulling the
paste and reflow. 6 Hrs. is the typical industry maximum.
We found very little problems with solder paste age, at least with the type
of paste we use. There was no corrilation to tombstoning.
Doing a process study on it, we found that the only thing we found was old
solder paste (in 6 + months of cold storage) has little effect, and 6+
months of room temp storage gives more mid chip solder balls, as the solder
beads oxidize some and don't coalesce as well. Leaving the paste on the
stencil for long periods (a few days) in the tests also did not cause any
problems other then the paste drying out some (which kneading it brought it
back to usable), although it could cause some moisture absorption which can
cause some problems with solder voiding in BGA spheres. We even tested a few
boards by printing, placing parts, and letting them sit in an uncontrolled
environment (an unused top shelf in production) in increments to up to 6
months after the first tests of a few weeks showed no significant changes.
What we found was the tackiness mostly disappeared and the paste hardened a
bit over the period. Solder balling increased over the 6 moth period, and
wetting was not quite as good, but acceptable. Fine pitch had no problems.
